Composite material has a life cycle much like other materials. Inspection is part of the process used to track the condition of composite materials during its life cycle.
Inconsistencies in the resin of a composite structure may be difficult to detect using nondestructive testing without accompanying inconsistencies in the fibers of the composite. Inconsistencies in the resin of a composite structure may be caused by many sources including, but not limited to, exposure to high temperature for short time periods or moderate temperatures for long periods, lightning strikes and electrical arcing.
At present, there are no approved nondestructive test methods to assess inconsistencies in the resin of composites, particularly thermally induced resin inconsistencies.
The alteration of the composite structure may not be visible to the human eye. Accordingly, in order to determine whether a composite structure or part may have been subjected to excessive thermal loads, it was necessary to cut sample plugs or sections from regions of the structure or part suspected of having undergone alteration. The material samples were subjected to any of several testing techniques, such as thermal mechanical analysis (TMA) or differential scanning calorimetry (DSC). The removal of sample plugs for analysis was a time consuming and destructive process, which, in the case of commercial aircraft operations, increased maintenance costs and cycle times at maintenance facilities.
Nondestructive testing techniques employing an inductive probe have been developed to inspect the mechanical properties of a composite material structure. Inductive-type probes rely on inductive (magnetic) coupling with the composite material in order to assess the mechanical properties of the material. Some composite material structures incorporate an electromagnetic shielding layer (or layers) near one face which is used to help protect an aircraft against lightning strikes. Inductive coupling probes suffer signal-to-noise losses when inspecting composite structures through electromagnetic shielding layers which act to divert circulating eddy currents so that the currents cannot adequately interact with the resin matrix.
Accordingly, there is a need for a method and apparatus for nondestructively inspecting carbon fiber reinforced resin composite structures which overcome the problem discussed above. Embodiments of the disclosure are directed toward satisfying this need.